In their search for renewable fossil-free fuels, Kevin Sivula’s team of chemical engineers at the Ecole Polytechnique Federale de Lausanne (EPFL), in collaboration with Toyota Motor Europe, took inspiration from the way plants are able to convert sunlight into chemical energy using carbon dioxide from the air in the phenomenon known as photosynthesis. The engineers invented a solar-powered artificial leaf that is capable of harvesting water from the air for conversion into hydrogen fuel. The results are published in the latest issue of Advanced Materials.
Their ingenious yet simple system combines semiconductor-based technology with novel gas diffusion electrodes, which have two key characteristics: they are porous, to maximise contact with water in the air; and transparent, to maximise sunlight exposure of the semiconductor coating. Instead of building electrodes with traditional layers that are opaque to sunlight, their substrate is actually a 3D mesh of felted glass fibre. When the transparent electrodes are coated with a light-harvesting semiconductor material, they do indeed act like an artificial leaf, harvesting water from the air and sunlight to produce hydrogen gas. The sunlight’s energy is stored in the form of hydrogen bonds.
“Developing our prototype device was challenging since transparent gas diffusion electrodes have not been previously demonstrated, and we had to develop new procedures for each step,” said Marina Caretti, the lead author of the work.
Research groups have previously shown that it is possible to perform artificial photosynthesis using a device called a photoelectrochemical (PEC) cell. It is generally known as a device that uses incident light to stimulate a photosensitive material, like a semiconductor, immersed in a liquid solution to cause a chemical reaction. But , this process has its disadvantages. It is hard to make large-area PEC devices that use liquid.
But Sivula’s team has demonstrated that PEC technology can be adapted for harvesting humidity from the air by using its new transparent gas diffusion electrode. Electrochemical cells (for example, fuel cells) have already been shown to work with gases, but the gas diffusion electrodes used previously are opaque and incompatible with solar-powered PEC technology.
In order to make transparent gas diffusion electrodes, the researchers start with a type of glass wool, which is essentially quartz fibre, and process it into felt wafers by fusing the fibres together at high temperature. Next, the wafer is coated with a transparent thin film of fluorine-doped tin oxide, known for its excellent conductivity, robustness, and ease to scale up. These first steps resulted in a transparent, porous, and conducting wafer, essential for maximising contact with the water molecules in the air and letting photons through. The wafer is then coated again, this time with a thin film of sunlight-absorbing semiconductor materials. This second thin coating lets light through but appears opaque because of the large surface area of the porous substrate.
The scientists built a small chamber containing the coated wafer and a membrane to separate the hydrogen gas produced for measurement. When the chamber is exposed to sunlight under humid conditions, hydrogen gas is produced.
While the scientists did not formally study the solar-to-hydrogen conversion efficiency in their demonstration, they acknowledged that it was modest for this prototype, and currently less than what is achievable in liquid-based photoelectrochemical (PEC) cells. The maximum theoretical solar-to-hydrogen conversion efficiency of the coated wafer is 12 per cent, whereas liquid cells have demonstrated an efficiency of up to 19 per cent.
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